Electret articles—that is, dielectric articles that exhibit at least quasi-permanent electric charge—are known to exhibit good filtration properties. The articles have been fashioned in a variety of constructions, but for air filtration purposes, the articles commonly take the form of a nonwoven polymeric fibrous web. An example of such a product is the Filtrete™ brand furnace filter sold by the 3M Company. Nonwoven. polymeric electret filters are also used in personal respiratory protection devices—see, for example, U.S. Pat. No. 5,307,796 to Kronzer et al., U.S. Pat. No. 5,804,295 to Braun et al., and U.S. Pat. No. 6,216,693 to Rekow et al.
A variety of methods have been used to make electrets, including fiber/electric particle bombardment (U.S. Pat. No. 4,215,682 to Kubik et al.), direct current “DC” corona charging (see, U.S. Pat. Re. 30,782 and 32,171 to van Turnhout and U.S. Pat. No. 4,592,815 to Nakao), hydrocharging (see, U.S. Pat. Nos. 5,496,507, 6,119,691, 6,375,886, and 6,783,574 to Angadjivand et al., U.S. Pat. No. 6,406,657 to Eitzman et al., and U.S. Pat. No. 6,743,464 to Insley et al.), and from exposure to polar liquids (U.S. Pat. No. 6,454,986 to Eitzman et al.). The electric charge that is imparted to the dielectric article is effective in enhancing particle capture.
During use, electret filters frequently become loaded with particles and contaminants that interfere with the filtering capabilities of the electret filter. Liquid aerosols, for example, particularly oily aerosols, may cause electret filters to lose their electret-enhanced filtering efficiency (see, U.S. Pat. No. 6,627,563 to Huberty).
Numerous methods have been developed to counter this filtering efficiency loss. One method includes adding additional layers of nonwoven polymeric web to the filter. This approach, however, can increase the pressure drop across the electret filter and can add to its weight and bulk. When the electret filter is used in a personal respiratory protection device, these drawbacks can be particularly troublesome. Increased pressure drop, for example, results in increased breathing resistance, making the respirator more uncomfortable to wear.
Another method for improving resistance to oily-mist aerosols, includes adding a melt processable fluorochemical additive such as a fluorochemical oxazolidinone, a fluorochemical piperazine, or a perfluorinated alkane to the polymer during the creation of the polymeric fibrous article—see, for example, U.S. Pat. Nos. 5,025,052 and 5,099,026 to Crater et al. and U.S. Pat. Nos. 5,411,576 and 5,472,481 to Jones et al. The fluorochemicals are melt processable, that is they suffer substantially no degradation under the melt processing conditions that are used to form the fibers in the electret web—see also U.S. Pat. No. 5,908,598 to Rousseau et al. In addition to a melt-processing method, fluorinated electrets also have been made by placing a polymeric article in an atmosphere that contains a fluorine-containing species and an inert gas and then applying an electrical discharge to modify the surface chemistry of the polymeric article. The electrical discharge may be in the form of a plasma such as an AC corona discharge. The plasma fluorination process causes fluorine atoms to become present on the surface of the polymeric article. The fluorinated polymeric article may be electrically charged using, for example, the hydrocharging techniques mentioned above. The plasma fluorination process is described in a number of U.S. patents to Jones/Lyons et al.: U.S. Pat. Nos. 6,397,458, 6,398,847, 6,409,806, 6,432,175, 6,562,112, 6,660,210, and 6,808,551. Other publications that disclose fluorination techniques include: U.S. Pat. Nos. 6,419,871, 6,238,466, 6,214,094, 6,213,122, 5,908,598, 4,557,945, 4,508,781, and 4,264,750; U.S. Publications US 2003/0134515 A1 and US 2002/0174869 A1; and International Publication WO 01/07144. U.S. Pat. No. 7,244,291 to Spartz et al. and U.S. Pat. No. 7,244,292 to Kirk et al. describe fluorinated electret articles that exhibit improved thermal stability.
U.S. Pat. No. 5,147,678 assigned to the University of Western Ontario describes the use of remote plasmas to modify the surfaces of polymeric articles. Remote plasma treatments are different from direct plasma treatments in that the sample surface is positioned away from the plasma creation region. The remote location causes the sample to be exposed to only the longest lived plasma species, which are able to reach the sample, unlike a broader ranger of species that are present in a direct plasma process. Remote N2, H2, and O2 plasmas are used to incorporate nitrogen and oxygen on a polymer surface.
U.S. Pat. No. 6,197,234, assigned to Conte S A, discloses the uses of a remote nitrogen plasma to treat polymeric powders or articles and describes introducing NF3, either upstream or downstream of the plasma zone, to increase “the anti-wettability of a body.”
Inagaki, from Shizuoka University in Japan, has authored several publications that describe the use of remote plasma sources to create modified polymer surfaces. One Inagaki article (N. Inagaki, S. Tasaka, and S. Shimada, J. APPL. POLYM. SCI. 79, 808-815 (2001)) describes surface modification of PET film by an argon plasma, and examines the surface modification created as a function of the distance from the “argon plasma zone”. Reported surface analysis finds oxygen added to the surface of PET treated in the plasma, but less oxygen added to the surface treated by the remote plasma. Another Inagaki article (Y. W. Park, N. Inagaki, J. APPL. POLYM. SCI. v. 93, pp. 1012-1020 (2004)) describes the surface modification of fluorinated polymer films using remote plasmas fed with Ar, H2, and O2. On three different fluoropolymer substrates (PTFE, ETFE, and PVDF), these remote plasma treatments reduced the surface fluorine concentration and increased the surface oxygen concentration.
A number of additional patents and publications describe plasma devices and methods, including remote plasma fluorination—see, for example, U.S. Pat. Nos. 5,147,678, 6,197,234, 6,477,980, 6,649,222, 6,819,096, 7,005,845, 7,161,112, 7,245,084, 7,445,695, and 7,468,494. U.S. Patent Publication 2007/0028944A1 describes a method of using NF3 and a remote plasma for removing surface deposits. International Publication WO03/051969A2 describes a plasma treatment to fluorinate porous articles. The following non-patent related publications also describe remote plasma techniques: Renate Foerch et al. Oxidation of Polyethylene Surfaces by Remote Plasma Discharge: A Comparison Study with Alternative Oxidation Methods, JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY, v. 28, pp. 193-204 (1990); N. Inagaki et al., Comparative Studies on Surface Modification of Poly(ethylene terephthalate) by Remote and Direct Argon Plasmas, JOURNAL OF APPLIED POLYMER SCIENCE, v. 79, pp. 808-815 (2001); and Brigitte Mutel, Polymer Functionalization and Thin Film Deposition by Remote Cold Nitrogen Plasma Process, JOURNAL OF ADHESION SCIENCE AND TECHNOLOGY, v. 22, pp. 1035-1055 (2008).